Method for inverse tone mapping of an image

Patent metadata

Field	Value
Publication number	US-9911181-B2
Application number	US-201415108562-A
Country	US
Kind code	B2
Filing date	Dec 2, 2014
Priority date	Dec 27, 2013
Publication date	Mar 6, 2018
Grant date	Mar 6, 2018

Abstract

Method comprising, for each pixel, obtaining a pixel expansion exponent value (E′(p)) by low pass filtering luminance values of colors of pixels in its spatial neighborhood, obtaining a pixel luminance-enhancement value (Y enhance (p)) by extraction of high frequencies of luminance values of colors of pixels in its spatial neighborhood, and then inverse tone mapping the luminance (Y(p)) according to the equation Y exp (p)=Y(p) E′(p) ×[Y enhance (p)] c .

First claim

The invention claimed is:

1. A method for inverse tone mapping of an image, the colors of which are represented in a color space separating luminance from chrominance, comprising:
obtaining a pixel expansion exponent value E′(p) for each pixel (p) of said image, by low pass filtering luminance values of colors of pixels in the spatial neighborhood of said pixel, obtaining a pixel luminance-enhancement value Y enhance (p) for each pixel (p) of said image, by extraction of high frequencies of luminance values of colors of pixels in the spatial neighborhood of said pixel,
inverse tone mapping the luminance Y(p) of each pixel (p) of said image into an expanded luminance Y exp (p) obtained through the product of the luminance Y(p) of said pixel at the power of the pixel expansion exponent value E′(p) obtained for this pixel and of the pixel luminance-enhancement value obtained for said pixel at the power of an exponent parameter c, which is superior or equal to 1.

2. The method for the inverse tone mapping of an image according to claim 1 , wherein said pixel expansion exponent values E′(p) are also obtained by reshaping said low-pass filtered luminance values such that these values are redistributed such that the mean expanded luminance of the image that is obtained is approximately equal to the mean luminance of the original image.

3. The method for the inverse tone mapping of an image according to claim 1 adapted to be reproduced on a display device having a given peak luminance D max , wherein said pixel expansion exponent values E′(p) are also obtained by rescaling said low-pass filtered luminance values such that the maximum luminance max(Y) over pixels of said image at the power of the pixel expansion exponent value obtained for the pixel having this maximum luminance max(Y) is approximately equal to said peak luminance D max .

4. The method for the inverse tone mapping of an image according to claim 1 , wherein said pixel expansion exponent values E′(p) are obtained by reshaping said low-pass filtered luminance values through a quadratic function applied to these values.

5. The method for the inverse tone mapping of an image according to claim 1 , wherein, for computing the pixel expansion exponent value of each pixel, said low pass filtering is performed by computing the product of a first Gaussian function applied in said spatial neighborhood of said pixel and of a second Gaussian function applied in the neighborhood of the luminance value of the color of said pixel.

6. The method for the inverse tone mapping of an image according to claim 5 , wherein, for said computing pixel luminance-enhancement values, said extraction of high frequencies of luminance values is performed by computing a ratio of a second low pass filtering divided by a third low pass filtering, wherein the third low pass filtering is configured to remove more details in the image than the second low pass filtering.

7. The method for the inverse tone mapping of an image according to claim 1 , wherein the luminance of said colors are encoded into increasing levels of wavelet decomposition, each level having at least a high-frequency coefficient (LH) and a low-frequency coefficient (LL), wherein the pixel expansion exponent value of each pixel is computed as be being equal to the low-frequency coefficient A(p)=LL of the highest level encoding the luminance of the color of said pixel (p), then performing said low pass filtering in said spatial neighborhood of said pixel.

8. The method for the inverse tone mapping of an image according to claim 7 , wherein, for said computing of the pixel luminance-enhancement value of a pixel, said extraction of high frequencies of luminance values of pixels located in the neighborhood of said pixel is performed by computing a difference between the luminance Y(p) of said pixel and said low-frequency coefficient A(p) of said pixel.

9. An image processing device for the inverse tone mapping of an image, the colors of which are represented in a color space separating luminance from chrominance, comprising: a low pass filtering module configured for obtaining a pixel expansion exponent value for each pixel of this image, by low pass filtering luminance values of colors of pixels in the spatial neighborhood of this pixel, a reshaping/rescaling module configured for reshaping/rescaling the low pass filtered luminance values provided by the low pass filtering module, a high frequencies extraction module configured for obtaining a pixel luminance-enhancement value for each pixel of the image, by extraction of high frequencies of luminance values of colors of pixels in the luminance neighborhood of this pixel, an inverse tone mapping module configured for inverse tone mapping the luminance of each pixel of the image into an expanded luminance obtained through the product of the luminance of said pixel at the power of the pixel expansion exponent value provided for this pixel by the reshaping/rescaling module and of the pixel luminance-enhancement value provided for said pixel by the high frequencies extraction module at the power of an exponent parameter c which is superior or equal to 1.

10. An electronic device comprising the image processing device according to claim 9 .

11. The electronic device according to claim 10 further comprising a display device having a given peak luminance D max and configured for displaying said inverse tone mapped image and a module configured for obtaining said pixel expansion exponent values E′(p) by rescaling said low-pass filtered luminance values such that the maximum luminance max(Y) over pixels of said image at the power of the pixel expansion exponent value obtained for the pixel having this maximum luminance max(Y) is approximately equal to said peak luminance D max .

12. A non-transitory processor readable medium having stored thereon a computer program product at least one of readable by a computer and executable by a processor, said computer program product comprising program code instructions for inverse tone mapping of an image, the colors of which are represented in a color space separating luminance from chrominance comprising:
obtaining a pixel expansion exponent value E′(p) for each pixel (p) of said image, by low pass filtering luminance values of colors of pixels in the spatial neighborhood of said pixel, obtaining a pixel luminance-enhancement value Y enhance (p) for each pixel (p) of said image, by extraction of high frequencies of luminance values of colors of pixels in the spatial neighborhood of said pixel, and
inverse tone mapping the luminance Y(p) of each pixel (p) of said image into an expanded luminance Y exp (p) obtained through the product of the luminance Y(p) of said pixel at the power of the pixel expansion exponent value E′(p) obtained for this pixel and of the pixel luminance-enhancement value obtained for said pixel at the power of an exponent parameter c, which is superior or equal to 1.

13. The image processing device of claim 9 , wherein the reshaping/rescaling module is further configured for reshaping low pass filtered luminance values such that these values are redistributed such that the mean expanded luminance of the image that is obtained is approximately equal to the mean luminance of the original image.

14. The image processing device of claim 9 , wherein the reshaping/rescaling module is further configured for rescaling low pass filtered luminance values such that the maximum luminance max(Y) over pixels of said image at the power of the pixel expansion exponent value obtained for the pixel having this maximum luminance max(Y) is approximately equal to a peak luminance D max , whe

Assignees

• Thomson Licensing

Inventors

• Pouli Tania
• Reinhard Erik
• Chamaret Christel

Classifications

• H04N5/57 Primary

  Control of contrast or brightness · CPC title

• H04N23/741

  by increasing the dynamic range of the image compared to the dynamic range of the electronic image sensors · CPC title

• G06T7/90

  Determination of colour characteristics · CPC title

• G06T2207/20064

  Wavelet transform [DWT] · CPC title

• G06T5/007 Primary

  Physics · mapped topic